An analogy to electrical pulse-width modulation exists in terms of fluid flow, wherein one or more valves control(s) a fluid flow in a cyclic manner, the valve or valves being operated in such a way that the lowest possible pressure drop is achieved when it/they is/are in the open position. Ideally, valves used in pulse-width modulation have just two states; that is to say either fully open (on) or fully closed (off). This is also in accordance with electrical pulse-width modulation, in which electrical switches, generally in the form of transistors, are fully on or fully off. A variable flow is achieved by the relationship between the opening time and the closing time being varied, but the frequency, as a rule, being kept constant. The relationship between the time in the open state and the time in the dosed state is usually termed the duty cycle, often denoted by the symbol “D”, and is given in percent. During cyclic operation at a given operating frequency, the duty cycle is independent thereof and only says something about the relationships mentioned between the off/on intervals. The fluid flow achieved will then, in the main, be proportional to the duty cycle of the valve (and, correspondingly, of the switch in an electrical context). At a duty cycle of 0% (D=0%), at which the valve is fully closed, there is no fluid flow. At D=50%, then 50% of the total fluid flow available is achieved, depending on the remaining resistance and the supply pressure in the circuit, et cetera.
FIG. 1 shows a function diagram for a pulse-width-modulated circuit with different duty cycles.
For the variable adjustment of a fluid flow, it is also common to use a form of choke/throttle valve. The use of a choke valve entails a relatively large loss when it is partly open. The loss is generally in the form of an isenthalpic pressure drop and accompanying free expansion and/or friction resulting from turbulent flow phenomena arising because of narrow or other fluid-flow-restraining passages in the fluid path, it all depending on the character of the throttling and the fluid. A valve operates with minimal loss only when the valve opening is large and the pressure drop across the valve is small at full fluid throughput. The port openings and a possible valve element slot/opening of a pure off/on-valve are dimensioned according to the expected or necessary fluid flow, so that the valve will exhibit only small losses when fully open, whereas for a proportional valve or other type of valve intended to provide variable fluid flow, there will always be considerable losses at partial flow, that is to say when the position of the valve is in a state between fully open and fully closed.
For smaller applications that require small fluid flows, this is not necessarily a problem. The problem will arise only when high losses in the form of pressure drops arise because of large fluid flows, and, in such cases, it could then be of great advantage to make use of a pulse-width-regulated valve instead, as the valve will then exhibit considerably lower loss according to the explanation above.
For heat engines, and then in particular steam engines and variants thereof, it is common to make use of a form of pulse-width modulation/regulation, and then often defined as cut-off-regulated injection. This form of pulse-width modulation makes the working fluid, often steam, be injected under full pressure into the expansion chamber, often cylinder chamber, of the engine until the chamber has reached a certain volume. The steam supply is then shut off (cut-off), and the steam goes on expanding near-adiabatically until the exhaust valve is opened as the cylinder chamber has reached a nearly full stroke volume. In this way, the steam supply may be regulated without any particular throttling, which would otherwise have entailed substantial losses.
In prior art there are many valve solutions for controlling such a process. In earlier steam engines, the cut-off point, which in turn gives the duty cycle of the supply valve, was regulated by the stroke of a slide valve being adjustable in the moving direction, among other things, and in that way, an adjustable cut-off was achieved. This gave great advantages over engines based on throttle valves, as explained above. Such engines could also, with simplicity, be reversed by a suitable valve gear mechanism being used. An example of a valve gear mechanism that could provide both controllable cut-off adjustment and also reversal is the Stephenson mechanism. This was usually used for steam locomotives, and innumerable other corresponding mechanisms have been made, such as the Walschaerts mechanism, the Corliss mechanism and, more recently, the Caprotti mechanism. Depending on the type, these could control everything from slide valves to partially rotating valves and seat valves with functions for variable cut-off and reversal.
What has nevertheless been a problem in several of them is achieving a fast enough acceleration of the valve elements or possibly sufficiently short opening/closing times when switching states. Because of the specific design solutions, it is often so that the movements of the valves around the switching points (opening/closing) starts from or ends in a standstill; that is to say, in these areas, the valve elements exhibit low speed, with the consequence that they provide a considerable throttling for a period when switching states.
In several valve gear mechanisms, the functioning is often such that there is a considerable throttling of the total opening of the valves when, in a cyclic state of operation, they are near the shutting-off state. This means that even though the valve mechanisms provide a practical approach to pulse-width modulation, the valves actually operate with considerable losses in consequence of the throttling they exhibit when switching between the closed and open states.
In addition, it is difficult to achieve a sufficiently low bottom limit for the duty cycle; that is to say, cut-offs and consequently duty cycles down towards 0% are difficult to achieve, especially without an element of throttling with accompanying loss. In particular, cut-off points adjusted below 5-10%, for example, can be difficult to achieve, which means that steam engines are difficult, partly impossible, to regulate for low power draws.